An inverter is an electronic circuit for converting direct current (DC) to alternating current (AC). Inverters are used in a wide range of applications, from small uninterruptible power supplies for a computer to large electric power transport utility applications, and variable speed industrial drives. A common application is for DC voltage source utilization such as a system designed to provide 115 volts AC from the 12 volts DC source provided in an automobile, the 12 volts DC source is first boosted over 200 V DC by a DC/DC converter, and then an inverter converts the high DC voltage to 115 volts AC. An inverter supplies AC power to operate equipment that is normally supplied from a power line or to power an AC electric motor. Inverters are also used to provide a source of AC power from fuel cell or photovoltaic solar cell power supplies. Uninterruptible power supplies are another type of application. Uninterruptible power supplies may use batteries to store power and an inverter to supply AC power from the batteries when power lines are not functioning or unavailable. When the power lines are restored, the batteries are recharged. High-voltage direct current power transmission is another application, where AC power is rectified into a high voltage DC and transmitted to another location. At the receiving location, an inverter converts the DC back to AC.
The term inverter has its origin from electromechanical converters. Historically, DC-to-AC power conversion was accomplished using electromechanical rotary converters coupling an AC electric motor to a DC electric generator in order to convert an AC input into a DC output. If the connections to a converter are inverted, the input is DC and the output is AC, so the historical inverted converter gave rise to the term inverter.
These early electromechanical devices were replaced with vacuum and gas filled tube switches in inverter circuits. Because they have higher voltage and current ratings, transistor semiconductor switches that can be turned on and turned off by means of control signals have become the preferred switching components for use in inverter circuits. In particular, the insulated gate bipolar transistors (IGBTs) are, at present, the most commonly used transistors for high current application such as for hybrid vehicles.
Three-phase is a common type of AC that can be produced by an inverter and used for electric power applications. It is a type of multi-phase system used to power motors, transformers, and many other devices. Three-phase has properties that make it very desirable in electric power systems: The phase currents sum to zero in a properly balanced load making it possible to eliminate the AC neutral conductor; power transfer into a balanced load is constant reducing generator and motor vibrations; and three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors. Three is the lowest phase order with these properties.
An important type of three-phase load is an electric motor. A three-phase electric motor has a simple design, high torque at low RPM, and high efficiency. Three-phase motors are used for pumps, fans, blowers, compressors, electric and diesel-electric locomotives and many other kinds of motor-driven equipments. Three-phase motors are more compact, less expensive, vibrate less, last longer than a single-phase motor of the same power rating, and are subsequently preferred over single-phase for motors above 10 HP (7.5 kW). Hybrid, fuel cell, and electric vehicles often use three-phase motors because their high starting torque can be used to accelerate a vehicle to a useful speed. A three-phase motor can also be used as a generator for regenerative braking.
Hybrid, fuel cell, and electric vehicles often have more than one source of DC power. For example, these vehicles may use electric batteries, ultra-capacitors, fuel cells, and fuel powered generators, all producing DC power. Moreover, the different sources will often have different voltages requiring voltage conversion to combine their electrical voltages. A traditional approach is to use a DC/DC converter, which is a complex and expensive piece of equipment.
Accordingly, it is desirable to have a simple system for using two sources for a three-phase inverter without using a DC/DC converter. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.